DC inductors are used as passive components in a DC link of AC electrical drives. A known practice is to use two separate inductors, one on DC positive and the other on DC negative bus bars. This approach is the size and mass of the inductors. There are also known cases of using single core inductors, which have two windings wound on the same core and each of them is meant to carry currents either on the DC positive or DC negative bus bars. In addition to the above, such a single core inductor can have a drawback because of a very high coupling coefficient between two windings. If some abnormal phenomenon occurs on the DC positive bus bar, then it can be automatically reflected on the negative DC bus bar, and vice versa. DC inductors can be used as filters for reducing harmonics in line currents in an input side rectifier system of an AC drive.
The use of permanent magnets in the DC inductors can allow for minimizing a cross-sectional area of the inductor core, thereby saving core and winding material and the needed space. The permanent magnets can be arranged in the core structure in such a way that a magnetic flux or the magnetization produced by the permanent magnets is opposite to that obtainable from the coil wound on the core structure. The opposing magnetization of the coil and permanent magnets makes the resulting flux density smaller and thus enables smaller cross-sectional dimensions in the core to be used.
As is known, permanent magnets have an ability to become de-magnetized if an external magnetic field is applied to them. This external magnetic field has to be strong enough and applied opposite to the magnetization of the permanent magnet for permanent demagnetization. In the case of a DC inductor having a permanent magnet, demagnetization may occur if a considerably high current is led through the coil and/or if the structure of the core is not designed properly. A current that may cause demagnetization may be a result of a malfunction in an apparatus to which the DC inductor is connected.
Known DC inductors with permanent magnets are based on core structures that have either permanent magnets inside a core magnetic gap or are specifically designed to hold the magnets with projecting structures or the magnets are directly attached to the outer surface of the structure designed specifically to use the permanent magnets. An example of a DC reactor is shown in EP 0744757 B1, where the permanent magnets are attached to the outer surface of the structure or inside the winding window.
Known DC inductors which include permanent magnets to the core structure or inside the core structure can be complicated and insecure. Additionally, extra back yokes are used for a permanent magnet return flux. The permanent magnet pieces are also quite fragile and do not tolerate mechanical impacts. Further, the inductance provided by one core structure is not easily modified in the existing inductors with permanent magnets. This is because if permanent magnet dimensions need to be modified, the whole inductor core structure or at least part of it should be modified.